In U.S. Pat. No. 5,003,394 of W. A. Lagoni, entitled "DYNAMIC VIDEO SYSTEM INCLUDING AUTOMATIC CONTRAST AND "WHITE STRETCH" PROCESSING" which issued Mar. 26, 1991, the advantages of using a "white stretch" processing section in a television system are disclosed. White-stretch processing emphasizes or boosts the amplitudes of mid-range luminance amplitudes relative to high luminance amplitudes as a function of a control signal representing the average picture level (APL) of a displayed image in order to enhance the over-all contrast of displayed images.
In allowed U.S. patent application Ser. No. 398,847 filed by W. A. Lagoni Aug. 25, 1989, and entitled AMPLIFIER ARRANGEMENT FOR PROVIDING A CONTROLLABLE NON-LINEAR TRANSFER CHARACTERISTIC USEFUL FOR IMPROVING THE CONTRAST OF AN IMAGE, there is described a specific implementation of white stretch processing suitable for use in Lagoni's patented system. In an exemplary embodiment the Lagoni white stretch processing circuitry comprises a source of luminance input signal and a source for providing a control signal representative of the average picture level (APL) of displayed images. A first circuit means responsive to the luminance input signal provides a first luminance output signal linearly related to the luminance input signal and provides a second luminance output signal non-linearly related to the luminance input signal. Means are provided for applying the first and second luminance signals to respective inputs of a "soft switch" or "blending" circuit which combines the first and second luminance signals signals in proportions determined by the APL representative control signal for providing a processed luminance output signal exhibiting greater non-linearity for smaller values of the APL control signal and exhibiting smaller non-linearity for greater values of the APL control signal. Advantageously, by this means, the Lagoni system provides enhanced contrast of displayed images.